Carbon-based sorbents such as activated carbon are currently used for controlling vapor phase mercury emissions in coal-fired power plant flue gases or waste incineration flue gases. In a typical coal-fired power plant, the configuration consists of a boiler, where water is evaporated to steam, followed by a steam super-heater section, a steam re-heater section, an economizer section (where the boiler feed-water is pre-heated), and finally an air heater, where the combustion air is pre-heated (FIG. 1). The combustion flue gas is cooled as it passes through each of these sections, transferring its heat to the water/steam stream on the other side of the heat exchangers. In a typical application, carbon sorbents are injected in the flue gas duct upstream of a particulate removal device such as a fabric filter or an electrostatic precipitator (e.g., the particulate collection device in FIG. 1), usually downstream of the air pre-heater and before the particulate removal device. The activated carbon used for such injection is manufactured off-site from carbonaceous materials like coal or coconut shells.
When powdered activated carbon was used in various applications with differing flue gas compositions, it was discovered that the efficiency of mercury capture varied significantly. For example, in the case when a low-halogen containing coal is combusted, the flue gases generated therein have a low concentration of halogen species such as HCl. In such cases, plain activated carbon performed poorly, i.e. the amount of material required to achieve desired capture efficiency was higher than when higher levels of halogen species were present in the flue gas. Methods to overcome this problem include adding a halogen component to the activated carbon sorbent prior to injection in the flue gas (see, e.g., U.S. Pat. No. 6,953,494). A method to add the halogen component as a separate stream from the activated carbon has also been used to address this issue The literature shows data on the improved performance of halogen-treated carbon for low-halogen content flue gas applications versus a plain activated carbon (see, e.g., U.S. Pat. No. 6,953,494).
Another problem with currently disclosed and used mercury control sorbents is their inability to perform with high efficiency in high concentration sulfur-containing flue gas. Kang et al. disclose results from using halogen-treated activated carbons for mercury control in relatively high sulfur concentration flue gases and discovered that a significantly higher quantity (almost 5 to 10 times) of activated carbon is required for achieving larger eduction in mercury emissions compared to cases where the flue gases had low concentration of sulfur species such as sulfur dioxide and sulfur trioxide. The coal sulfur concentration was 2.5 percent in the high sulfur case compared to about 0.4 percent sulfur in the coal in the low sulfur case.
These data show that not only does a lack of halogen components in the flue gas, but also the presence of sulfur species in the flue gas, adversely affect the performance of activated carbon in removing mercury species from the flue gas.
In another application, sulfur trioxide is injected into the flue gas upstream of a electrostatic precipitator (ESP) to improve and thereby achieve acceptable particulate capture performance. The injected sulfur trioxide (an ESP conditioning agent) adsorbs on the fly ash particles, lowering the resistivity of the collected dust in the precipitator to within an ideal range of values, thereby improving overall precipitator performance. However, Dillon et al. showed that SO.sub.3 conditioning impairs mercury capture. For example, with no SO.sub.3 injection and an activated carbon (brominated) injection rate of 4 lbs of carbon per million actual cubic feet of flue gas (MMacf), 75-90 percent mercury reduction was obtained. At the same sorbent injection rate, and with 5.4 ppm of SO.sub.3 in the flue gas for improved ESP performance, only 35-50% mercury reduction was obtained. In this case, the sorbent was injected around 300-350.degree. F., downstream of the air pre-heater and upstream of the ESP. When the sorbents were injected upstream of the air pre-heater, at around 650-750.degree. F., at an injection rate of 5 pounds of sorbent per million cubic feet of flue gas, an 85 percent reduction in mercury concentration was achieved compared to about 60 percent reduction when the same quantity of sorbent was injected downstream the air pre-heater.
These data show that there is some benefit for mercury capture by injecting the sorbents upstream of the air pre-heater in a coal-fired boiler, as opposed to downstream of the air heater.
Srinivasachar and Kang (US Patent Application No. 20050039598 and US Patent Application No. 20090056538) describe a method for removing mercury from the products of fuel conversion comprising: disposing carbonaceous sorbent into contact with the products of fuel conversion at a contact location having a temperature between 400.degree. F. and 1100.degree. F., whereupon the carbonaceous sorbent adsorbs mercury; and removing the carbonaceous sorbent having mercury adsorbed thereon. They also describe injecting the sorbent upstream of the air heater. See, also, Kang et al. 2007
Another application where mercury capture is difficult and requires large quantities of sorbent material to achieve high capture efficiencies is for coal-fired boilers equipped with a hot-side electrostatic precipitator. In this application, the coal-fired boiler configuration consists of an evaporator section, followed by a steam super-heater and re-heater section and an economizer section. The flue gases leaving the economizer are then directed to a hot-side electrostatic precipitator to remove the particulates before sending the “clean” flue gas to an air pre-heater, where heat is transferred from the hot flue gas to the combustion air, which is then routed to the boiler. In this configuration, sorbent, such as activated carbon or halogenated activated carbon, is injected upstream of the hot-side ESP at around 500 to 700.degree. F. Because of the short residence time for contact between the sorbent and the flue gases in such a configuration, mercury capture performance is poor. For example, with the injection of brominated activated carbon at 5 pounds per million actual cubic feet of flue gas, only 60 to 70% mercury removal was achieved in flue gas generated from a Powder River Basin coal (sub-bituminous, low sulfur).
There is a need for improved mercury control performance in hot-side ESP applications.
Yet another problem with using activated carbon sorbents for mercury control is that when the spent carbon is mixed with the ash in the particulate collection device, it renders the ash unusable for some end-applications such as concrete. This is because when fly ash is used in concrete manufacturing, if it has certain components that adsorb the hydrophobic air entraining agents that are used in concrete manufacturing, then it is rejected for such end use. Activated carbon because of its surface area and its propensity to adsorb the air entraining agents is deleterious. A foaming index test is used to evaluate the suitability of the ash for use in concrete: if the foaming index is below a critical value, then the ash is suitable for use in concrete.
Kang et al. (2007) provide data for the impact of activated carbon injection upstream of an air heater at around 800.degree. F. where the foam index value of the resulting ash increased from about 150 with no sorbent injection to around 500 with the injection of approximately 0.8 pounds of sorbent per million actual cubic feet of flue gas (density calculated at 300.degree. F.). This sorbent injection rate was required to achieve 90 percent mercury removal. The acceptable foam index value was around 250 for commercial fly ash sales, and so the injected sorbent proved to be deleterious for subsequent use of the ash for concrete applications. Note that foam index values indicated above were provided in units unique to their measurement technique and were to be considered only on a relative basis.
There is a need for methods to mitigate the deleterious effects of activated carbon-based sorbents on ash, when the ash is used in end applications such as concrete.
Biermann et al. (U.S. Pat. No. 6,974,564) disclose a clay and limestone byproduct from the papermaking industry as an adsorbent for mercury, which is injected into the high temperature region flue gas (around 2000.degree. F.). The injection rates for 95 percent capture are in the range of 20 lb/MWh a factor of 20 larger than typical activated carbon injection and 0.8-3.2 lb/MWh, for 75 percent reduction, a factor of 4 larger typical carbon injection rates. The high injection rates are likely to be expensive and also may affect the performance of the power plant components by depositing and fouling their surfaces. Also the injection of large quantities of material may adversely affect the performance of particulate collection devices and may result in increased particulate emissions.
Chang et al. (U.S. Pat. No. 6,558,454) describe a process for removing vapor phase contaminants from a gas stream that includes the step of adding a “raw carbonaceous starting material” into a gas stream having an “activation temperature” sufficient to convert the raw carbonaceous starting material into an “activated material” in-situ. The “raw carbonaceous starting material” can be either a solid-phase, liquid phase or vapor-phase material. The “activated material” then adsorbs the vapor phase contaminants (e.g. mercury), and the “activated material” containing the vapor phase contaminants is removed from the gas stream using a particulate collection device. The process further claims an injecting step, where said injecting step includes the step of injecting said “raw carbonaceous starting material” into said gas stream wherein said activation temperature of said gas stream is between about 100.degree. C. and about 1400.degree. C. In addition, the gas stream residence time, which is the amount of time the “raw carbonaceous starting material is present in the gas stream” into which it is injected before conversion to an “activated material” was indicated to be about 0.1 to 30 seconds.
The process above is not very effective as it uses a raw carbonaceous material as the material for injection. Therefore, the produced sorbent within the flue gas is not optimum, as it does not have a high activity and a high surface area.
Chang et al. identify the process of producing “activated carbon”, the preferred sorbent for sorption of trace contaminants such as mercury from fluid streams, as normally being carried out in large rotary kilns with treatment time of several hours. An object of their invention was to provide a method to generate an alternative “activated material” separate from “activated carbon” which was indicated to be expensive. While Chang et al. used a gas stream for contacting the raw carbonaceous material with temperatures in the range of 100 to 1400.degree. C. for the purposes of activation, they did not anticipate the beneficial effects of injecting an engineered material such as activated carbon at high temperatures and did not identify a preferred temperature range for the purposes of removing the vapor phase contaminant such as mercury using activated carbon.
Per the embodiments of Chang et al. (U.S. Pat. No. 6,558,454) described in FIGS. 4 and 5 of the patent, (Column 7; lines 20 to 30), the raw carbonaceous material is injected via an injector into the boiler and at a high temperature to activate the raw material, but the “activated material,” that is activated externally to the boiler in these embodiments, is injected into the “exhaust stream from the boiler,” likely at much lower temperatures around 300.degree. F. and not within the boiler.
For the above reasons, Chang et al. does not provide an optimum temperature range for contacting activated carbon with gas streams containing contaminants such as mercury to maximize mercury capture by the activated carbon.
Powdered activated carbon (PAC) can be injected into the flue gas in a coal-fired power plant at several locations. One such location is the region upstream of the air heater at temperatures between 500 and 800.degree. F. PAC is typically injected with air as the carrying medium and through lances that penetrate completely into the flue gas duct to ensure good distribution of the PAC with the flue gas.
During the process of injection at high temperature locations, the carbon particles are subjected to high temperatures as well as high oxygen concentrations within the injection lances. Consequently, the carbon particles can start oxidizing and burning within the injection lances, deteriorating the sorbent and potentially even destroying them, as well as potentially causing other operational problems such as local deposition, plugging and over-heating. This is one additional challenge of using activated carbon sorbent for mercury control while injecting the sorbent at high temperatures.